1. Field of the Invention
The present invention generally relates to a transporting device and image forming apparatus. More specifically, the present invention relates to a transporting device that transports a flat sheet type object such as a document and paper sheet or the like onto which a document image is to be transferred, and an image forming apparatus provided with the transporting device, and even more specifically relates to a transporting device having an elevator plate mounted on an object mounting surface to be liftable and lowerable, and an image forming apparatus.
2. Background Information
Image forming apparatuses such as copiers, facsimile machines, scanners, and multipurpose apparatuses are provided with document feeding devices. An image forming apparatus consecutively reads each document by automatically transporting a document having a plurality of sheets one by one to an image reading position using the document feeding device. The document feeding device receives a stack of document sheets so as to abut a pick-up roller in order to separate one sheet at a time from the multiple sheets of a document placed in a document tray. Since the thickness of the stack of document sheets changes according to the number of sheets of the document stacked in the document tray, the distance separating the pick-up roller and the document mounting surface must be adjusted in accordance with the thickness of the document stack to abut suitably the document stack. This adjustment is accomplished using the elevator plate mounted on the mounting surface of the document to be liftable and lowerable. The document stack on the elevator plate is raised so as to abut the pick-up roller when the elevator plate is actuated. For example, The elevator plate has a support shaft on the upstream side of the pick-up roller in the document transport direction, and the elevator plate is oscillated on this shaft via a motor. Thus, the leading edge of the uppermost sheet of the document abuts the pick-up roller and the pick-up roller feeds the document sheet.
The stopping position of the elevator plate when the document is being fed is detected by an upper limit position sensor (hereinafter referred to as “upper limit switch”). The upper limit switch detects whether or not the top surface of a document stack abuts the pick-up roller. For example, the stopping position can be detected by the upper limit switch detecting the position of the top surface of the document stack in a document feeding device that has a configuration in which the position of the pick-up roller is stationary. Furthermore, the stopping position can be detected by the upper limit switch detecting the position of the pick-up roller or a member that moves in linkage with the pick-up roller in a document feeding device provided with a mechanism lifting the pick-up roller together with the lifting of the elevator plate.
The thickness of the document stack decreases as the document feeding progresses. In the case of a document having many sheets, the pick-up roller may not feed the lower document sheets when the thickness of the document stack decreases. Therefore, when, for example, the thickness of a document stack is reduced below a standard amount in such a document feeding device, the elevator plate is actuated and the document stack is lifted. Thus, the top surface of the document stack is caused to abut suitably the pick-up roller, and table document feeding can be realized. The lifting of the elevator plate while a document is being fed is performed in concert with the change in the detection status of the upper limit switch described above.
When a document is loaded on the elevator plate, however, the elevator plate is disposed at a document loading position. The document loading position is at the lower limit of the elevator range of the elevator plate. The movement of the elevator plate to the document loading position is performed when an immediately previously performed document feeding has been completed. That is, the elevator plate is driven downward when a sensor for detecting the presence/absence of a document on the document tray enters the negative detection state, i.e., no document sheets present. The elevator plate is lowered until the elevator plate position is detected by a lower limit position sensor (hereinafter referred to as “lower limit switch”) provided at a position opposing the bottom surface of the elevator plate as having reached the lower limit.
In the document feeding device provided with an elevator plate described above, the amount of movement of the elevator plate is measured to detect the abnormal raising and lowering of the elevator plate. For example, when a stepping motor is used as a lift motor for lifting the elevator plate, the amount of movement of the elevator plate can be measured by counting the number of drive pulses that drive the lift motor, for example, as shown in Japanese Laid-Open Patent Publication Nos. 2000-53255, 2000-95358, and 2005-187145.
When a document stack which is near the upper limit capacity of the document tray is loaded on the elevator plate in the document feeding device mentioned above, the pick-up roller abuts the top surface of the document stack the moment the document has been loaded. At this time, document feeding can begin under the existing condition without driving the elevator plate since there is no need to raise the document stack. When the thickness of the document stack decreases as the document feeding progresses, the elevator plate is lifted to raise the document stack as previously described.
The lifting of the elevator plate while a document is being fed in the manner mentioned above is performed in concert with the change in the detection status of the upper limit switch. That is, the upper limit switch is set to the detection state (ON condition) when the top surface of the document stack abuts the pick-up roller, and the upper limit switch is set to the negative detection state (OFF condition) when the thickness of the document stack is reduced a predetermined amount. The lifting of the elevator plate starts in conjunction with these states such that the elevator plate is lifted until the upper limit switch is turned ON. At this time, the necessary amount of elevation is the amount of decrease in the document stack, and may be a fixed amount unrelated to the thickness of the document stack. However, while this same document stack is being fed, the first lifting operation of the elevator plate requires more time than the second and subsequent lifting operations of the elevator plate despite the fact being lifted a fixed amount. This phenomenon is undesirable inasmuch as processing performance is reduced. In addition, when an abnormal lift is detected as an amount of movement that exceeds an amount of a slight surplus added to the fixed amount, the document feeding device detects an abnormal lift since a greater number of drive pulses are input in the first lifting operation of the elevator plate compared to the second and subsequent lifting operations. Conversely, when the first lifting operation of the elevator plate is set as a normal lift, an abnormal lift generated in the second or subsequent lifts cannot be detected.
The lifting of the elevator plate while a document is being fed is changed by the detection via the upper limit switch, and can be realized, for example, by inputting a drive pulse to drive the lift motor a fixed amount. However, in this configuration the amount by which the elevator plate is lifted the first time is less than the amount by which the elevator plate is lifted the second and subsequent times. In this case, the elevator plate is not lifted a desired amount during the first lifting operation, and the top surface of the document stack does not suitably abut the pick-up roller. Therefore, more time may be required for a retry process, or a feed error may occur.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved transporting device and image forming apparatus. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.